1. Field of the Invention
The present invention relates to methods and apparatus for assessing the ventilatory status of a patient. More particularly, the present invention provides a system for implementing a non-invasive procedure for estimating the amount or concentration of dissolved carbon dioxide within the arterial portion of the vasculature. The arterial carbon dioxide content, expressed as a partial pressure, i.e., pCO.sub.2, is an important measure of ventilatory status which ultimately reflects pulmonary health.
2. State of the Art
Physicians and other health care providers often use elevated arterial pCO.sub.2 (PaCO.sub.2) as an indicator of incipient respiratory failure. In this regard, the determination of PaCO.sub.2 is useful in optimizing the settings on ventilators and detecting life-threatening blood gas changes in an anesthetized patient undergoing surgery. The traditional method for obtaining arterial blood gas values is to extract a sample of arterial blood and measure the partial pressure of carbon dioxide using a blood gas analyzer (PaCO.sub.2.sup.ABG). Arterial puncture has inherent limitations: 1) arterial puncture carries a degree of patient discomfort and risk, 2) handling of the blood is a potential health hazard to health care providers, 3) significant delays are often encountered before results are obtained and, 4) measurements can only be made intermittently.
Continuous invasive monitoring requires in-dwelling arterial lines which entail inherent problems. These include sepsis, slow response times, and signal decay. The nature of this monitoring system excludes its use under routine care and is generally restricted to intensive care units within a hospital facility.
There have been attempts to assess PaCO.sub.2 levels indirectly, including a technique known as capnography. The approach utilized in capnography involves tracking patient exhalation and measuring expiratory gas CO.sub.2 concentration against time during one or more respiratory cycles. The resulting relationship is plotted to create a graph depicting three distinct phases in breath CO.sub.2 gas concentration during the patient's exhale cycle. (See, FIG. 1). Typically, the three phases reflect the clearing of the conducting airways which do not normally participate in gas exchange (i.e., airway dead space) (Phase I) followed by the exhalation of air from conducting airways dynamically mixed with lung gases from the active (alveoli) membrane surfaces within the lung that have undergone gas exchange with arterial blood (Phase II). The final phase (Phase III) reflects the exhalation of unmixed gas from regions of the lung which are normally in active exchange with the alveoli tissue. Phase III thus closely resembles (in healthy patients) gas properties associated with arterial blood in contact with the lung for gas exchange (CO.sub.2 release and O.sub.2 absorption). In normal lungs, the graph line of Phase III is substantially level (slope=0) since ventilated and perfused alveolar regions are closely matched. In a diseased lung, the Phase III line may not appear level due to a mismatch in ventilation and perfusion of this lung region. See, Table I below:
TABLE I Phase I Represents CO.sub.2 -free gas expired ftom the airway conduction structures where gas exchange does not occur, Phase II The S-shaped upswing in CO.sub.2 concentration (expressed as a percent) represents the transition from airway to alveolar gas, and Phase III The alveolar plateau representing CO.sub.2 rich gas from the alveoli.
In the past, capnography has utilized the peak or end-tidal (PetCO.sub.2) values as an estimate of PaCO.sub.2. PetCO.sub.2 is a measure of the mean alveolar partial pressure of carbon dioxide from all functional gas exchange units of the lung. PetCO.sub.2 obtained from capnography is a measure of mean alveolar pCO.sub.2, which value approximates PaCO.sub.2 in normal lungs. Because CO.sub.2 readily diffuses across the alveolar-capillary membrane, the PetCO.sub.2 closely approximates the PaCO.sub.2 with normal ventilation-perfusion. The difference between PetCO.sub.2 and PaCo.sub.2 is primarily a function of the proportion of the lung where gas exchange does not occur (Fletcher, R., Johnson, G., and Brew, J., The Concept of Deadspace with Special Reference to Single Breath Test for Carbon Dioxide. Br. J. Anaesth., 53, 77, 1981). In patients afflicted with a lung disease, there often exists a proportional increase in the region of the lungs where gas exchange does not occur. This increase in so-called "alveolar dead space" results in a significant difference between peak CO.sub.2 (PetCO.sub.2) obtained from capnography and elevated arterial CO.sub.2 (PaCO.sub.2).
Other techniques have been utilized for assessing patient blood gas levels with mixed results. Transcutaneous sensors measure tissue pCO.sub.2 diffused through the heated skin but have practical and theoretical limitations. Oximetry is a widely used, non-invasive method for estimating the arterial oxygen carried on hemoglobin. For example, U.S. Pat. Nos. 4,759,369, 4,869,254 and 5,190,038 describe pulse oximeters which measure the percentage of hemoglobin which is oxygenated. However, neither of the aforementioned techniques measures the amount of dissolved oxygen present, nor the amount of oxygen carried when hemoglobin levels are reduced. Low hemoglobin levels are found when there is a significant blood loss or when there is insufficient red blood cell formation. Additionally, oximeter readings are specific to the point of attachment, which is typically the finger tip or ear lobe, and may not reflect the oxygen level of vital organs during conditions such as shock or hypothermia.
There remains a significant need in the art for an accurate, non-invasive, sensitive method for accurately determining the levels of arterial blood gases. As will be seen hereinafter, the instant invention sets forth a non-invasive system to overcome the problems of the prior art.